In recent years, attention is focused on various techniques for utilizing power effectively. For example, there is an approach to avoid, as much as possible, cut off of the power supply line having high priority (Patent Document 1: International Publication No. 05/093924). There is also an approach to prevent the flow out of stored power to a system without restricting the power generated from a solar cell (Patent Document 2: Japanese Patent Laying-Open No. 2002-171674).
Moreover, there is an approach (hereinafter, also referred to as first prior art) to prevent reverse power flow that is the flow of power from a facility that requires power (hereinafter, also referred to as “power demanding facility”) to a power system that supplies power to the power demanding facility (Patent Document 3: Japanese Patent Laying-Open No. 2004-208426). The power demanding facility is a residence, for example. The power system is a system that supplies power such as at an electric power company.